On-Board Oxygen Generation Systems (OBOGS) utilizing Pressure Swing Adsorption (PSA) technology have been known in the art to generate breathable, oxygen-enriched product gas. PSA systems generally utilize molecular sieve material, such as zeolite, to separate incoming air from an air source, such as engine bleed air of an aircraft, by adsorbing nitrogen from the bleed air while allowing oxygen to pass therethrough. The separated oxygen may then be ultimately directed to specific areas (e.g., cockpit, cabin) and personnel (e.g., pilot, crew, passengers) aboard the aircraft so as to provide a breathable gas. The adsorbed nitrogen may be periodically purged from the zeolite under reduced pressure by using pressure swing techniques in a known manner. The purged nitrogen is then either dumped overboard or used for other purposes, such as inerting the fuel tank ullage of the aircraft.
To improve system operations and efficiencies, multiple bed systems may be used, such as a dual bed system wherein a first bed is actively separating oxygen from engine bleed air while the second bed is regenerating the zeolite under reduced pressure. A switching valve assembly may dictate which bed is receiving engine bleed air and which bed is regenerating. In this manner, once the air separation efficiency of the first bed is no longer sufficient to produce oxygen enriched output gas having a desired purity of oxygen, the switching valve assembly may direct the pressurized engine bleed air into the regenerated second bed while permitting the corrupted first bed to regenerate its zeolite bed at reduced pressure.
OBOGS efficiency issues can arise when contaminating moisture and water/chemical vapors from the pressurized inlet air enter the molecular sieve bed and interfere with the zeolite active sites and thereby cause the gas separation efficiency of the molecular sieve bed to decrease. By way of example, moisture may damage the crystalline structure of the zeolite. Moreover, this moisture may also be very difficult to desorb from the zeolite bed during the regeneration phase, particularly if the moisture has travelled deep into the center of the bed. As a result, when zeolite particles in a bed are damaged or have adsorbed liquid water, they are much less effective at adsorbing nitrogen such that the air separation efficiency of the molecular sieve bed is compromised.
When nitrogen is not adsorbed by the zeolite and passes through the bed (what may be known in the art as “nitrogen breakthrough”), it will be entrained within the output product gas and act to reduce the oxygen percentage thereof. Nitrogen breakthrough is sometimes desired, such as in the case where a lower oxygen concentration is desired (for instance, depending on altitude). In these instances, a specific concentration of nitrogen may be allowed to pass through the OBOGS so as to produce a product gas possessing the desired percentage of oxygen. However, unintentional and/or uncontrolled nitrogen breakthrough can have disastrous results in that aircraft personnel may not be receiving the desired oxygen percentage and may suffer serious and potentially deadly health effects, such as hypoxia or hypoxemia. Thus, should nitrogen breakthrough be desired, it must be done in a controlled manner in order to maintain the desired oxygen concentration in the separated gas, while also avoiding harmful adverse health effects.
An OBOGS may also use the source air to drive pneumatic valves (directly or through electrically-driven pilot solenoid valves). In multiple bed systems, these valves may be used to selectively cycle source air through the molecular sieve beds as described above. These valves may also assist in the calibration of an on board oxygen sensor. However, moisture in the source air may subject the pneumatic valves and/or solenoid valves to corrosive damage. Moist air may also prevent accurate calibration of the oxygen sensor while also exposing the sensor's internal circuitry to potential corrosive damage.
It may thus be appreciated that it is undesirable to have moisture and water/chemical vapors in the source air. Several proposed solutions have been developed to address this issue. One such proposed solution is to include one or more coalescing filters within the air separation system before the air separation unit so as to reduce any moisture that may enter the zeolite beds. Coalescing filters generally function by causing moisture and vapor particulate (e.g., droplets) to coalesce on a borosilicate glass filter or its equivalent. The particulates aggregate together as condensation on the thin exterior of the glass filter and, when having sufficient density, gravity forces the condensation to trickle into a drain positioned below the filter. Coalescing filters are not, however, usually effective for certain aircraft applications. For instance, cyclic pressure swinging within the aircraft tends to force the particulate to prematurely fall off the glass filter and be subsequently carried into the molecular sieve beds within the flow of inlet air. Coalescing filters are also ineffective in filtering moisture and vapor particulate that have passed over the borosilicate glass filter during the PSA vapor phase, allowing condensation to occur at the molecular sieve inlet and sieve bed. Furthermore, coalescing filters can be rendered temporarily ineffective when placed in an upside down orientation, such as when the corresponding aircraft makes an inverted maneuver.
Another proposed solution is the use of centrifugal separators. Centrifugal separators generally separate contaminating moisture and oil particulate from the bleed air by forcing the airflow to travel centripetally within the separator. Centrifugal forces cause the denser water and/or oil particulates to move to the outer wall of the separator where the particulates will collect and move to a drainage port. While efficient at removing dense contaminants, centrifugal separators are not effective in removing particulates and vapors should these contaminants have insufficient density to be forced to the separator's outer wall prior to being discharged. Rather, these non-separated contaminates remain in the inlet air such that moisture may enter the sieve beds.
Yet another proposed solution is to incorporate a thin desiccant material layer over the molecular sieve bed, an example of which has been disclosed in U.S. Pat. No. 6,681,807 to Byrd (the '807 patent), the entirety of which is incorporate herein. As disclosed within the '807 patent, a layer of desiccant material (e.g., activated alumina) may be deposited on the surface of the zeolite bed wherein the desiccant material adsorbs moisture and vapor particulates from the inlet gas airflow prior to the inlet gas entering the molecular sieve. However, one drawback to this approach is that the molecular sieve is in close contact with the desiccant and may pull moisture from the desiccant when the system is not in operation due to the sieve bed having a higher affinity for moisture than the desiccant. Moreover, the thin desiccant material layer may not allow enough for airflow residence time within the desiccant material so as to enable adsorption of moisture. Increasing the depth of the desiccant layer would increase the airflow residence time but would also cause the desiccant to encroach upon the surrounding molecular sieve components.
A further proposed solution has been to incorporate a vessel of desiccant upstream from a molecular sieve in a radial bed configuration. In this type of system, a mixed bed absorber may include a vessel positioned parallel with an absorbent bed. During operation, a pressurized airflow of wet source air enters into the vessel where the airflow is dried by beds of alumina beads. The dried airflow may then pass through the absorptive material of the air separation (zeolite) bed where contaminating gasses, such as nitrogen and carbon dioxide, are removed before the separated airflow is discharged. While such a system may enable drying of the source gas, this design requires regeneration of the alumina beads through a continuous counter-current flow of heated regeneration gas. This, in turn, causes the source air to intermix with the regeneration gas before the source air makes downstream contact with the radial bed. As a result, separation of additional contaminating gasses not from the airflow of source gas is required, thereby leading to decreased separation efficiencies and decreased separator operational lifetimes. Counter-current flow also creates airflow resistance within the vessel which can cause additional stresses in the absorber. The implementation of heated regeneration gas also results in an undesirable amount of energy usage. Furthermore, the short distance between the vessel and radial absorbent bed does not provide for a physical gap large enough to ensure residual airflow moisture does not make contact with the absorptive material or the radial absorbent bed. Since bed thickness is generally determined by a minimum residence time of the contaminated gas, radial bed absorbers also typically require larger volumes of absorbent material than other molecular sieve configurations. As such, this system may not be suitable for aviation applications where reduced size and weight are critical design parameters.
Another proposed solution to alleviate input airflow moisture has been to cool the incoming air (such as hot engine bleed air) through at least one heat exchanger before the airflow enters the OBOGS unit. According to this method, the moist air is cooled which allows water and/or oil vapors to condense to a liquid that can then be separated from the airflow and drained. However, while some water and water/oil vapor may be removed, the airflow continues to be saturated with unwanted vapors when exited from the heat exchanger. That is, the relative humidity of the air does not change as the cool air merely holds less water vapor than hot air. As a result, water/oil vapors may still be carried into the molecular sieve where they may condense within the sieve bed and cause a reduction in air separation efficiency.
Thus, there remains a need for a system and method which removes moisture from source air, such as source air for use in an OBOGS. There is also a need for providing a moisture-free airflow to the valves used to cycle the molecular sieve beds within the OBOGS. The moisture-free airflow may also assist in oxygen sensor calibration. The present invention satisfies these, as well as other, needs.